There is a great open source project on codeplex: .NET validation framework.
This project apply reusable and customizable validation rules to properties, methods, and parameters in your business objects. Works with WPF, ASP.Net, Windows Forms, ASP.Net MVC and web services.
(This library is written in C# but can be consumed by any .net assembly)
Requires: .net Runtime 3.0.
I really recommend to use this framework it is very strong and easy to use.
Want to read more? here is the link to codeplex: .NET Validation Framework.
I really wants to see your comments on this project, I use it on my ASP.NET applications.